NZ260812A

NZ260812A - Fibre optic amplifier with monitor transducers coupled to output ports of four part coupler

Info

Publication number: NZ260812A
Application number: NZ260812A
Authority: NZ
Inventors: Henning Bulow; Jurgen Otterbach
Original assignee: Alcatel Australia
Priority date: 1993-07-01
Filing date: 1994-06-22
Publication date: 1996-11-26
Also published as: CA2126404A1; EP0637860B1; AU6479394A; DE59400677D1; EP0637860A1; JPH07147565A; DE4321856A1; AU679514B2; ES2095111T3; JP3485967B2; US5442479A

Abstract

In the case of fibre-optic amplifiers, various parameters are measured, for example the input power, in order to make assessments regarding the operating mode of the transmission link and of the fibre-optic amplifier itself. In known fibre-optic amplifiers, a part of the input power is extracted through a coupler (2) for this purpose. In the case of transmission of optical signals at low power, such as for example in the case of digital signals, the extracted part of the input power can be detected only with high expense. In the fibre-optical amplifier according to the invention, the radiation produced by the amplified spontaneous emission (ASE) is extracted and evaluated in order to monitor the pump power and input power. Assessments can thereby be made regarding the state of the input signal.

Description

<div class="application article clearfix" id="description"> 260 8 1 2 Priority Date(s): .U.Xl.f* 3 Complete Specification FSed: Class: (6) Publication Date: 2 -6 NOV -1986 P.O. Journal No: NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION (v OFFICE 2 2 JUN 1994 RE.CDVED " FIBRE-OPTIC AMPLIFIER" WE, ALCATEL AUSTRALIA LIMITED, ^Cnj ooo OO« A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a fiber-optic amplifier. Fiber-optic amplifiers comprising four-port couplers are known, for example, from Auge J., et al, "Fortschritte im Bereich der optischen Verstarkung", Electrisches Nachrichtenwesen, 4th Quarter 1992, pages 37 to 44. There a fiber-optic amplifier is shown which has two pump-light sources, two pump-light couplers, and a length of optical waveguide doped with erbium ions. In fiber-optic transmission systems, fiber-optic amplifiers serve to amplify optical signals which are guided in the optical waveguides. Through the pump light, the erbium ions contained in the length of optical waveguide, which are in an energetic ground state, are raised into an excited state, from which they revert, through either spontaneous or stimulated emission, to the ground state. The stimulated emission is caused by the optical signal travelling through the length of optical waveguide. If no optical signal is travelling through the length of optical waveguide, the transition to the ground state will be spontaneous, with the energy difference between ground state and excited state being converted into radiation which propagates along the length of optical waveguide. During this propagation in the length of optical waveguide, the spontaneous radiation is amplified. The radiation resulting from amplified spontaneous emission (ASE) emerges at the two ends of the length of optical waveguide. This radiation will hereinafter be referred to as "ASE". In the 260 8 1 2 case of erbium-doped fiber-optic amplifiers, the wavelength of this ASE is in the range around 1550 nm. It thus lies in the wavelength range of the optical signal. In such transmission systems it is important to have information on the transmission link and on the operating states of the fiber-optic amplifiers in order to safeguard the transmission itself and ensure the necessary transmission quality. This includes information as to whether input power is present for the fiber-optic amplifier and how hight the output power of the latter is. If no input power is present, this may point to a fiber break, for example. In that case it must be ensured that the transmission link is "eye-safe", i.e., that there is no potential hazard to the eye of a person looking, intentionally or unintentionally, into a broken optical waveguide. To guarantee this, the system or individual parts thereof must be shut down in the event of a fiber break. From the above literature it is known to monitor the input and output power of a fiber-optic amplifier in order to control the amplifier's gain. Portions of the input and output power are coupled out by couplers forming part of the fiber-optic amplifier, and detected by photodiodes. During the transmission of analog signals, such as TV signals, high signal power levels prevail. The coupled-out signal power can be detected with simple circuitry. 260812 Things are different if the signal power is lower, as is the case in digital transmission systems, for example. The coupled-out signal power is correspondingly lower, and the amount of circuitry required to detect this low signal power increases. It is an object of the invention to provide a fiber-optic amplifier in which even low input power levels can be monitored without the need for complicated and costly circuitry. According to the present invention, there is provided a fiber-optic amplifier for amplifying an optical signal, comprising a four-port coupler, wherein the reception of light in the wavelength region of the optical signc, an optical-to-electrical transducer is provided which is connected to a port of the coupler from which emerges a portion of the light which propagates in the fiber-optic amplifier in a direction opposite to that of the optical signal. According to a further aspect of the invention, there is provided a fiber-optic amplifier for amplifying an optical signal, comprising a four-port coupler and a first optical-to-electrical transducer which is connected to a port of the coupler from which a portion of the optical signal emerges, wherein the reception of light in the wavelength region of the optical signal, a second optical-to-electrical transducer is provided which is connected to a port of the coupler from which emerges a portion of the light which propagates in the fiber-optic amplifier in a direction opposite to that of the optical signal. 4 26 0 8 One advantage of the invention is that it is possible to clearly distinguish between a fiber break and a disconnected transmit laser. Another advantage is that the actual pump power can be monitored provided that the input is disabled. The invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of a fiber-optic amplifier; Figure 2 shows a second embodiment of a fiber-optic amplifier, and Figure 3 is a graph showing the ASE power detected at prot A3 as a function of the input power. Figure 1 shows the basic construction of a fiber-optic amplifier as far as it is relevant to the invention. Its amplifying portion 1, containing a doped length of optical waveguide 12 and a pump-light source 11, is shown schematically; more detailed representations are contained in the literature, e.g., Auge J. Any optical isolators and power supplied that may be present are not shown. An optical signal to be amplified enters the fiber-optic amplifier at an input 5 and leaves the amplifier as an amplified signal at an output 8. The amplification and direction of the optical signal are indicated by arrows of different size at the input and output of the fiber-optic amplifier. The fiber-optic amplifier further includes a coupler 2 with four ports A1 to A4, a first optical-to-electrical transducer 4, and a second optical-to- 26 0 8 1 electrical transducer 9. Such couplers are described, for example, in Grau, G., "Optische Nachrichtentechnik", zweite Auflage, Berlin, Heidelberg, New York, Tokyo, Springer Verlag, 1986, pages 296 to 302. The optical signal to be amplified is fed through the input 5 of the fiberoptic amplifier into port A1 of the coupler 2. Port A2 of the coupler 2 is connected to the amplifying portion 1, which amplifies the optical signal. The amplified optical signal is available at the output 8 of the fiber-optic amplifier, which is connected to the amplifying portion 1. Between the amplifying portion 1 and the output 8 a coupler and an optical isolator may be provided, which are not shown. The first optical-to-electrical transducer 4 is connected to port A3 of the coupler 2, from which emerges light that propagates in a direction opposite to that of the signal. The second optical-to-electrical transducer 9 is connected to port A4 or the coupler 2, from which emerges a portion of the light that propagates in the fiber-optic amplifier in the direction of the signal. The individual components are interconnected via optical waveguides L. For simplicity, the term "light" is used for any optical radiation, visible or invisible. The second embodiment of a fiber-optic amplifier, shown in Figure 2, differs from that of Figure 1 only in that the second optical-to-electrical transducer 9 is not present. 26 0 8 1 The electric signal appearing at the output of an optical-to-electrical transducer 4, 9 is evaluated in a microprocessor provided in the fiber-optic amplifier for electronic control. This is indicated by the arrow emanating from the transducer 4, 9. Figure 3 shows the dependence of the ASE power on the input power of the fiber-optic amplifier. The ordinate represents the ASE power Pase in JfW, which propagates in a direction opposite to that of the optical signal, and the abscissa represents the input power of the fiber-optic amplifier in //W. The ASE power decreases with increasing input power, as shown. In each of the embodiments shown, port A3 outputs a portion of the light which propagates in the fiber-optic amplifier in a direction opposite to that of the optical signal. This light is ASE, for example, but in addition, unabsorbed pump light may emerge from that port if the pump light is injected in a direction opposite to that of the optical signal. In that case, the pump light can be blocked by a wavelength-selective filter ahead of the optical-to-electrical transducer 4, so that only ASE will be detected. Port A4 of the coupler 2 outputs a portion of the light which propagates in the fiber-optic amplifier in the direction of the optical signal. The portion emerging from port A4 of this light is detected by the optical-to-electrical transducer 9. In the fiber-optic amplifiers mentioned at the beginning, this is used to monitor the input power. The detected signals are evaluated as follows: 260 8 Since ASE occurs only when the length of optical waveguide is being pumped, the pump power can be checked when the input power is intentionally cut off. If a high ASE power is measured with the input power cut off, this means that the pump source is operating properly. Figure 3 shows that an ASE power of 6500 jt/W is measured if the input power is zero. The ASE power decreases with decreasing pump power. The measurement of the ASE power provides not only yes-or-no information but also quantitative information on the input pov • . At an ASE power of 1000 p\N, the input power is approximately 100 //W. As shown in Figure 3, the ASE power is high at low input power levels. It is advantageous that this high ASE power is easier to detect than low input power. A particular advantage of the first embodiment (Figure 1) is that it permits the detection of a fiber break: If a fiber breaks before the input 5, part of the ASE (approximately 4% in the case of a straight fracture) will be reflected at the point of fracture, and the optical-to-electrical transducer 9 will falsely detect input power. If, however, the optical-to-electrical transducer 4 simultaneously detect a high ASE power, this means that the actual input power is very low or even zero. By evaluating the electric signals from the optical-to-electrical transducers 4, 9, a fiber break can be positively detected. If both the first optical-to- 26 0 8 electrical transducer 4 provide an electric signal greater than a threshold value, an evaluating device (microprocessor) will deliver a signal which indicates a fiber break. Based on this information, action can be taken in the transmission system, i.e., in the event of a fiber break, for example, the system can be shut down, wholly or in part. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 260812 What we claim is:

1. A fiber-optic amplifier for amplifying an optical signal, comprising a four-port coupler, wherein the reception of light in the wavelength region of the optical signal, an optical-to-electrical transducer is provided which is connected to a port of the coupler from which emerges a portion of the light which propagates in the fiber-optic amplifier in a direction opposite to that of the optical signal.

2. A fiber-optic amplifier for amplifying an optical signal, comprising a four-port coupler and a first optical-to-electrical transducer which is connected to a port of the coupler from which a portion of the optical signal emerges, wherein the reception of light in the wavelength region of the optical signal, a second optical-to-electrical transducer is provided which is connected to a port of the coupler from which emerges a portion of the light which propagates in the fiber-optic amplifier in a direction opposite to that of the optical signal.

3. A fiber-optic amplifier as claimed in claim 2, wherein an evaluating device is provided which indicates a fiber break if the first and second optical-to-electrical transducers provide an electrical signal greater than a threshold value. 260 8 1

4. A fibre-optic amplifier substantially as herein described will reference to Figures 1 - 3 of the accompanying drawings. ALCATEL AUSTRALIA LIMITED 10 B. O'Connor Authorized Agent P5/1/1703 N.2. p >rncE 22 mm REC&IV, 11 </div>